This invention is related to refractive eye surgery, and more particularly to a software tool for assisting surgeons in planning this surgery.
Refractive errors result when the optical elements of the eye, namely the cornea and the lens, do not focus a clear image onto the retina. An eye is considered emmetropic if it has no refractive error. Most eyes have at least some degree of refractive error. In myopia, the optical elements are too strong for the length of the eye, and the image is focused in front of the retina. In hyperopia, the optical elements are too weak for the length of the eye, and the image is focused behind the retina. In astigmatism, the optical elements cannot focus an image to a single point, and the image is split and focused at two separate points.
When a refractive error is present, a lens may be used to refocus light onto the retina. This lens may be in the form of a spectacle lens or contact lens. Additionally, a lens surgically implanted within the eye (intrastromal or intraocular) can be used.
The cornea is the strongest refracting lens of the eye. Therefore, small changes in the shape of the cornea result in large changes in the overall refractive properties of the eye. By making the cornea flatter or steeper in a controlled fashion, a surgeon can affect changes in the eye""s refractive power. Using mathematical calculations and review of surgical results, a surgeon can predict the amount of refractive change induced by a given amount of corneal reshaping. Surgeons use these calculations to predict the outcome of corneal refractive surgery.
Radial Keratotomy (RK), Arcuate Keratotomy (AK), and Photo Refractive Keratectomy (PRK) are refractive corneal surgical techniques that have been commonly used in the past to induce controlled changes in the shape of the cornea, and subsequently in the refractive error. In RK and AK procedures, surgeons use a diamond-bladed scalpel to make small incisions in the cornea. Radial incisions, used in RK, reduce myopic refractive errors while arcuate incisions, used in AK, reduce astigmatic refractive errors. These incisions induce changes in the corneal curvature and, consequently, alter the eyes"" refractive properties.
PRK procedures use a laser to reshape the corneal surface. The laser sculpts a thin layer, between 5 and 10 mm in diameter, on the corneal surface. This technique has many advantages over RK/AK since it usually cuts less than 10 percent of the way through the cornea, as opposed to about 90 percent with RK/AK, and can correct a wider range of myopic, hyperopic, and astigmatic refractive errors.
Laser Assisted In Situ Keratomileusis (LASIK) offers additional advantages over both RK/AK and PRK. The LASIK procedure consists of two distinct surgical procedures. The first part of the procedure involves the surgical creation of a corneal flap. The laser is then used to treat deeper corneal stroma tissue in much the same way as PRK treats the stroma near the corneal surface. The flap is then replaced after the laser treatment. This offers the advantage of leaving much of the corneal surface intact, leading to faster and more comfortable recovery for the patient.
Despite its advantages over other corneal refractive surgery techniques, LASIK refractive surgery still has a number of shortcomings. This invention addresses, among other things, one of these shortcomings, namely, the fact that many excimer lasers now used for LASIK were built for the purpose of performing PRK refractive surgery. Consequently, when a surgeon enters patient data such as refractive error and patient age, the laser calculates a treatment based on the expected results of PRK, not LASIK. Therefore, a surgeon must compensate for the fact that LASIK and PRK differ in refractive treatment effects by making appropriate adjustments to the refractive error information that is entered into the machine. This means that the surgeon often cannot use the patient""s actual refractive error to achieve the best surgical result. Instead, the refractive error information that is actually entered must be adjusted by as much as 25 percent in order to optimize the treatment. These adjustments are calculated using a nomogram that is created on the basis of previous surgical results. An example of a nomogram for myopia is shown in FIG. 1. There are different nomograms utilized for each type and combination of refractive errors including myopia, hyperopia, and astigmatism. It should also be understood that these charts are specific to particular surgical laser equipment, and somewhat dependent on variations in surgical technique. Different surgical laser equipment manufactured by different suppliers would therefore require different chart nomograms. The chart nomogram (FIG. 1) has patient age in years on one axis and diopters of refractive error on the other axis. Each axis contains a range for each data entry. Where the x and y-axis meet, a correction percentage is given. This represents the amount that the programmed refractive error must be changed in order to perform LASIK. A problem arises in that a large amount of surgeon judgment is required in utilizing a chart nomogram. This judgment is required because each axis includes range data that will affect the percent correction factor in setting the laser. For example, if the patient is at the low end of an age range on one axis and has a spherical equivalent at the low end of the refractive range on the other axis, the surgeon may adjust the percentage of correction suggested in the chart nomogram by a different amount than if the patient""s age or refractive error were at a different point within the same range on the x or y axis. The correction percentage indicates that the refractive laser should be set for the desired diopters of correction by adjusting the spherical refraction. The surgical plan may also include a simultaneous but different formula to adjust the cylindrical portion (astigmatism) of the manifest refraction.
The growing body of clinical evidence suggests that a nomogram must take into consideration a number of factors including but not limited to the refractive error of the eye and the patient""s age at the time of surgery. Other parameters may influence the refractive effect of the laser treatment on the eye, including the vertex distance of the refraction, and the patient""s gender. Despite this accumulation of data, LASIK, like all surgical procedures, can vary in effectiveness from surgeon to surgeon and from patient to patient. It should be emphasized that, as more information is collected, the nomograms can, and should, be revised to include statistical data indicative of a particular surgeon""s procedural results.
The beginning surgeon has no personal surgical data, and therefore cannot predict the effectiveness of an excimer laser, built and programmed for PRK, when used for LASIK. With experience, a surgeon may begin to understand the variables that will result in over- or under-correction, however, with no personal experience; a beginning surgeon must rely on nomograms based on the results of others. What is needed is a tool to assist such surgeons in developing a standardized technique and nomogram based on the results of other surgeons. What follows is a brief review of the decisions a surgeon must make in preparation for LASIK surgery.
Refractive Error Correction. One of the most important decisions involves understanding the design of the laser system being used. As stated, many lasers were designed and programmed for the purpose of performing PRK. If a surgeon wishes to perform LASIK using one of these machines, he or she must understand that the programmed correction must be adjusted based on the known differences between PRK and LASIK. These differences are currently addressed using published chart nomograms, however, more precise adjustment calculations would be desirable. Currently, the clinical evidence suggests that age and refractive error have a bearing on the effectiveness of LASIK treatment, but as more data become available, factors such as gender may also be shown to affect LASIK treatment.
Most patients wish to be emmetropic after LASIK surgery. That is, they would like to have both eyes corrected for good distance vision. Younger emmetropic patients, those under forty, have enough accommodative power in their lenses to allow them to focus on near objects. However, this accommodative power is naturally lost with age and individuals usually require glasses for reading and seeing close objects clearly. Older LASIK patients, especially those over age forty, may opt for monovision correction by having each eye corrected differently in order to have one xe2x80x9creading vision eyexe2x80x9d and one xe2x80x9cdistance vision eye.xe2x80x9d Understanding how to operate on such monovision patients requires that the surgeon determine the patient""s dominant eye as well as understand the patient""s reading needs. Monovision patients opt to leave one eye, usually the non-dominant eye, slightly nearsighted. This is achieved by intentionally undercorrecting an eye if it is initially myopic or over-correcting an eye if it is initially hyperopic. After surgery, the patient can avoid the need for glasses by shifting attention between eyes depending on the visual task.
Suction Ring Size. The first step of LASIK, creating the corneal flap, is done by fixating the eye using a suction ring. The suction ring also serves to apply controlled pressure to the globe, allowing for a more reproducible corneal flap cut. The surgeon may use a ring size of 8.5 millimeter or 9.5 millimeter or another size, depending on the steepness or flatness of the cornea and on whether the refractive surgery is being done to correct myopia, hyperopia or astigmatism, or a combination of astigmatism together with myopia or hyperopia. A flatter cornea is more reproducibly fixated using a larger diameter ring such as a 9.5-millimeter ring, whereas a steeper cornea is more reproducibly fixated using a smaller ring size such as an 8.5-millimeter ring. A larger ring size and the resulting larger corneal flap has the advantage of better accommodating a possible enhancement in the event of an initial over correction, but has the disadvantage of being more difficult to handle. A larger ring size, such as a 9.5-millimeter ring, is currently recommended for the treatment of hyperopic refractive errors. Plate Depth. The current state of the art requires the surgeon to select a plate to set the depth of the corneal flap cut, such as a plate of 160 or 180 microns. In making this decision, the surgeon must take into consideration the initial corneal thickness, the refractive error requiring surgical correction, monovision needs if any, patient""s age and the optical zone requiring treatment. Additionally, the surgeon may have to take into consideration other parameters, such as the vertex distance that the refraction is performed at, and gender, all of which may affect the depth of laser ablation. In addition, the surgeon must consider industry standards of minimal safe residual corneal bed depth as well as the likelihood of the need for an enhancement, in order to select the optimal plate thickness.
Although examples of plate depths of 160 and 180 microns have been described, it should be noted that the same information applies for cuts at other plate depths, and for developing techniques whereby a laser or other device is focused at a controlled depth to either make the corneal flap cut. Alternatively, the refractive laser or refractive device is itself focused at a controlled depth to perform the refractive surgery.
Unfortunately, residual corneal thickness and flap thickness are both best kept maximally thick, which are mutually exclusive goals. Surgeon judgment is required to select the plate depth that best balances these goals. Surgeon judgement is also required to avoid surgery on a cornea that is too thin for the degree of refractive correction required.
Laser Pulse Frequency. Currently, the surgeon selects the laser""s pulse frequency. The higher the frequency, the more rapidly the laser pulses occur, and the shorter the laser treatment time. Quicker laser treatments, in theory, result in fewer variations in corneal thickness due to time-related drying between pulses, and, consequently minimizes variability in refractive results. On the other hand, if high laser frequencies are used to treat small myopic refractive errors, the laser treatment time may be very short, increasing the risk that involuntary eye movements or loss of fixation during laser treatment will have a disproportionately large and undesirable effect.
What is needed is a method to reduce surgeon variability and balance the mutually exclusive needs of rapid treatment to avoid variations in ablation depth due to time related corneal thinning and drying, with the need to have a longer treatment time to minimize abnormal, inefficient and off-center treatment due to patient eye movement.
Patient Cautions. Current surgical standards do not provide surgeons with cautions to reduce adverse visual outcomes, such as those which may result from patients who are too young for surgery, corneas that are too thin for the desired correction, eyes that are excessively dry, pupils that are too large for a given refractive error treatment at a particular optical zone requiring treatment, and other patient-specific variables, such as discrepancies between corneal and refractive astigmatism, or non-patient specific errors such as data entry errors. What is needed is a tool for identifying these risk factors that may lead to an undesirable result.
It is therefore an object of this invention to provide tools, that assist surgeons in preparing a preoperative plan for refractive eye surgery. This and other objectives are achieved by providing a system and method which provide for receiving inputs from the surgeon based upon patient data and eye measurements, calculating precise corrective settings for the laser equipment utilized in the surgery, and outputting the precise corrective settings along with recommendations for the surgical procedure, including refractive error correction, plate thickness, ring size, and laser frequency. In addition, the system and method provide cautions with respect to a patient""s age, tear function, pupil size, corneal thickness, and discrepancies between refractive and corneal astigmatism.